Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Patent
1992-12-07
1994-08-09
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
G01R 3320
Patent
active
053370015
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a magnetic field generating assembly for use in nuclear magnetic resonance (NMR) apparatus.
Conventional NMR apparatus has a large solenoidal magnet which generates a high strength magnetic field within its bore with, at the centre of the bore, a working region in which the magnetic field strength is substantially uniform. To achieve the high strengths required for NMR imaging experiments, superconducting solenoids must be used with the superconducting material housed in large cryostats. Additionally, in order to perform an experiment a gradient coil system is provided, usually formed by resistive conductors, to superimpose magnetic gradients on the field within the working region. In operation, a sample to be examined is inserted into the bore of the magnet so that the region of the sample to be examined is located within the uniform working region and the experiment then performed. In order to allow maximum space for the sample, the gradient field system is positioned close to the inner surface of the main coils and this leads to a number of disadvantages. In particular, there is significant coupling between the gradient coils and the surrounding magnetic structure. For example, whenever the current in a gradient coil is changed, the flux threading conductive parts of the magnet structure changes so inducing an emf and an opposing eddy current. This reduces the speed at which each gradient change can be achieved and thus the speed of the overall experiment. Furthermore, large currents need to be supplied to the gradient coils in order to achieve useful magnetic field gradients and changes in these currents also lead to large eddy currents.
SUMMARY OF THE INVENTION
In accordance with the present invention, a magnetic field generating assembly for use in NMR apparatus comprises primary magnetic field generating means for generating a primary magnetic field which is substantially homogeneous in a working region, the primary magnetic field generating means being constructed such that an object to be examined can be introduced from outside the assembly into the working region in a direction transverse to the direction of magnetic flux through the working region; and gradient magnetic field generating means for imposing one or more magnetic field gradients through the working region, the gradient magnetic field generating means being movable relative to the primary magnetic field generating means for positioning locally to the position of an object to be examined in the working region.
A completely new form of magnetic field generating assembly has been devised which represents a new approach to the construction and use of NMR apparatus. The gradient magnetic field generating means (typically in the form of suitably designed coils) can be placed adjacent or within the working region itself and even in contact with the object being examined and so remote from the primary magnetic field generating means. This has a number of significant advantages:
1. The signal to noise ratio is improved by decreasing the separation between the gradient magnetic field generating means and the object;
2. The small volume defined by the gradient magnet field generating means and the open access of the primary magnetic field generating means removes most of the eddy current between the two;
3. The use of this open access type of magnetic field generating assembly in the case of human body NMR imaging allows a patient to be positioned with the main field transverse to his body allowing the use of solenoidal RF coils for providing the RF pulses necessary in an imaging experiment;
4. Sample specific RF coils can be designed to have a high Q factor for greater efficiency;
5. Strong gradient magnetic field generating means close to the object improves resolution;
6. Rapid gradient ramp rates are possible;
7. High patient comfort and low acoustic emissions are achieved which are particularly beneficial in the case of human body imaging;
8. Low cost components can be used for tradi
REFERENCES:
patent: 4689591 (1987-08-01), McDougall
patent: 4920316 (1990-04-01), Egloff
patent: 5146924 (1992-09-01), Sepponen
PCT Search Report dated Aug. 7, 1991.
Armstrong Alan G. M.
Hawkes Robert C.
McDougall Ian L.
E. I. du Pont de Nemours & Company
Oxford Medical Limited
Tokar Michael J.
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